Website Dating:My Density shows first and second degree relationships between websites.

The interface could be a bit cooler and smoother (perhaps if they used a bit of AJAX and ensured the text was easier to read on scroll overs if they are showing a ton of data in a small area), but it looks like a cool idea. more info about My Density and even more info here.

A9 OpenSearch:
I saw this mentioned a while ago, but I think I forgot to link to it. Essentially it allows search results to be reformatted & reappear elsewhere. Robin Good has more about why he feels it is important.